Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive to lower and lower wavelengths of radiation and has also led to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization.
High resolution, chemically amplified, deep ultraviolet (100–300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries. There are two major deep ultraviolet (uv)exposure technologies that have provided significant advancement in miniaturization, and these are lasers that emit radiation at 248 nm and 193 nm. Examples of such photoresists are given in the following patents and incorporated herein by reference, U.S. Pat. No. 4,491,628, U.S. Pat. No. 5,350,660, EP 794458 and GB 2320718. Photoresists for 248 nm have typically been based on substituted polyhydroxystyrene and its copolymers. On the other hand, photoresists for 193 nm exposure require non-aromatic polymers, since aromatics are opaque at this wavelength. Generally, alicyclic hydrocarbons are incorporated into the polymer to replace the etch resistance lost by not having aromatics present. Furthermore, at lower wavelengths the reflection from the substrate becomes increasingly detrimental to the lithographic performance of the photoresist. Therefore, at these wavelengths antireflective coatings become critical.
The use of highly absorbing antireflective coatings in photolithography is a simpler approach to diminish the problems that result from back reflection of light from highly reflective substrates. Two major disadvantages of back reflectivity are thin film interference effects and reflective notching. Thin film interference, or standing waves, result in changes in critical line width dimensions caused by variations in the total light intensity in the resist film as the thickness of the resist changes. Reflective notching becomes severe as the photoresist is patterned over substrates containing topographical features, which scatter light through the photoresist film, leading to line width variations, and in the extreme case, forming regions with complete photoresist loss.
In the past dyed photoresists have been utilized to solve these reflectivity problems. However, it is generally known that dyed resists only reduce reflectivity from the substrate but do not substantially eliminate it. In addition, dyed resists also cause reduction in the lithographic performance of the photoresist, together with possible sublimation of the dye and incompatibility of the dye in resist films.
In cases where further reduction or elimination of line width variation is required, the use of bottom antireflective coating provides the best solution for the elimination of reflectivity. The bottom antireflective coating is applied to the substrate prior to coating with the photoresist and prior to exposure. The resist is exposed imagewise and developed. The antireflective coating in the exposed area is then etched, typically in an oxygen plasma, and the resist pattern is thus transferred to the substrate. The etch rate of the antireflective film should be relatively high in comparison to the photoresist so that the antireflective film is etched without excessive loss of the resist film during the etch process. Inorganic types of antireflective coatings include such films as TiN, TiON, TiW and spin-on organic polymer in the range of 30 nm. Inorganic B.A.R.C.s require precise control of the film thickness, uniformity of film, special deposition equipment, complex adhesion promotion techniques prior to resist coating, separate dry etching pattern transfer step, and dry etching for removal.
Organic B.A.R.C.s are more preferred and have been formulated by adding dyes to a polymer coating (Proc. SPIE, Vol. 1086 (1989), p. 106). Problems of such dye blended coatings include 1) separation of the polymer and dye components during spin coating 2) dye stripping into resist solvents, and 3) thermal diffusion into the resist upon the baking process. All these effects cause degradation of photoresist properties and therefore are not the preferred composition.
Light absorbing, film forming polymers are another option. Polymeric organic antireflective coatings are known in the art as described in EP 583,205, and incorporated herein by reference. However, these polymers have been found to be ineffective when used as antireflective coatings for photoresists sensitive to 193 nm. It is believed that such antireflective polymers are very aromatic in nature and thus are too reflective, acting as a mirror rather than absorbers. Additionally, these polymers being highly aromatic, have too low a dry etch rate, relative to the new type of non-aromatic photoresists used for 193 nm exposure, and are therefore ineffective for imaging and etching. Photoresist patterns may be damaged or may not be transferred exactly to the substrate if the dry etch rate of the antireflective coating is similar to or less than the etch rate of the photoresist coated on top of the antireflective coating.
Therefore, it is necessary to have a bottom antireflective coating that functions well at exposures less than 230 nm. Such antireflective coatings need to have high etch rates and be sufficiently absorbing to act as antireflective coatings. U.S. Pat. No. 5,935,760 describes a bottom antireflective coating based on a very specific crosslinkable polyester polymer.
The novel antireflective coatings of the present invention, comprising a novel polyester polymer based on a unique chemical structure, have been found to have good dry etching properties, which enable a good image transfer from the photoresist to the substrate, and also good absorption characteristics to prevent reflective notching and line width variations or standing waves, particularly at 193 nm. The antireflective coating of the present invention has a relatively high etch rate such that the antireflective coating is removed with minimal loss in the thickness of the photoresist layer. Additionally, substantially no intermixing is present between the antireflective coating and the photoresist film. The antireflective coatings also have good solution stability and form particularly thin films with good coating quality, the latter being particularly advantageous for lithography. When the antireflective coating is used with a photoresist in the imaging process, clean images are obtained with good lithographic properties.